Hydroformylation reactions involve the preparation of oxygenated organic compounds by the reaction of synthesis gas (carbon monoxide and hydrogen) with carbon compounds containing olefinic unsaturation (hereinafter “olefinic material”). The reaction is generally performed in the presence of a hydroformylation catalyst such as cobalt or rhodium, and results in the formation of a product comprising an aldehyde which has one more carbon atom in its molecular structure than the starting olefinic feedstock. By way of example, higher alcohols useful as intermediates in the manufacture of plasticizers, detergents, solvents, synthetic lubricants, and the like, are produced commercially in the so-called Oxo Process (i.e., transition metal catalyzed hydroformylation) by conversion of C3 or higher olefin fractions (typically C5–C12) to an aldehyde-containing oxonation product having one additional carbon atom (e.g., C6–C13). Hydrogenation and distillation yields the respective alcohols, or the aldehydes may instead be further oxidized to the respective acids.
The Oxo Process to convert olefinic material to aldehydes generally proceeds through three basic stages as explained below by specific reference to a catalyst comprising cobalt.
In the first stage, or oxonation reaction, the olefinic material and the proper proportions of CO and H2 are reacted in the presence of a cobalt-containing carbonylation catalyst to give a product comprising predominantly aldehydes containing one more carbon atom than the reacted olefin. Typically, alcohols, paraffins, acetals, and other species are also produced in the hydroformylation reaction. The catalyst can be supplied to this section by numerous methods known in the art, such as by injecting cobalt acetate or cobalt formate directly or by supplying cobalt from a precarbonylation stage or catalyst makeup stage in the form of a cobalt anion (Co−1) or organically soluble form of Co+2, such as cobalt naphthalate, oleate, or cobalt oxides.
The oxygenated organic mixture from the oxonation (or oxo) reactor(s), which typically contains various salts and molecular complexes of the metal from the catalyst (i.e., the “metal values”) as well as the aldehydes, alcohols, acetals and other species, referred to as the crude aldehyde or crude hydroformylation mixture, is treated in a second stage, the demetalling stage. In the demetalling stage, typically a reaction is caused to separate the metal values from the aldehyde, such as by injecting dilute acetic and/or formic acid. This separation of the metal values is optionally helped by the additional injection of an oxidant. Various oxidants can be used including, but not limited to, oxygen, air or hydrogen peroxide, and can be used pure or diluted with inert diluents or inert carriers. The crude hydroformylation mixture separates into phases with the organic phase comprising the desired aldehyde separated from the aqueous phase comprising the cobalt as a salt exemplified by cobalt acetate and/or formate. The organic phase is sent to other unit operations downstream to be converted to the desired final product.
In the third stage of the Oxo Process the metal values removed in the second stage are worked up in a way that they can be reused in the oxonation section. There are several ways taught in the prior art to work up this catalyst. For example, one way is to convert the aqueous metal salt to an organically miscible compound such as cobalt naphthenate, and inject it directly into the oxonation reactor(s). Another way is to subject the aqueous salt solution in the presence of an organic solvent to high pressure synthesis gas, converting it to active carbonyl, and delivering it to the oxonation section via extraction, stripping or the like. It would be ideal if all of the cobalt is recovered and eventually passed in the proper form to the first stage described above.
These aforementioned three process stages may occur in more or less than three distinct vessels and numerous variations and improvements, including adding to, deleting from, or combining these stages, have been proposed over the years with various degrees of success. Although the use of dilute low molecular weight organic acid to retrieve the cobalt values as its corresponding cobalt salt has been known for many years, efficient recycling of cobalt salt in the Oxo Process has heretofore proved to be elusive.
U.S. Pat. No. 2,816,933 observes that the most direct method of utilization of cobalt acetate consists of recycling directly the aqueous cobalt stream from the demetalling stage to the primary aldehyde synthesis zone of the oxonation reaction stage. The problem with such a scheme is it introduces considerable quantities of water in to the reactor. Excess water substantially decreases the olefin conversion and may result in reactor flooding and complete loss of reaction. Instead, the patent teaches that after injection of sufficient acetic acid to combine with all the cobalt present in the demetalling stage, the entire mixture, including crude product, is allowed to separate into aldehyde and aqueous phases in a settler. After sufficient time, the lower aqueous phase containing cobalt acetate is passed to an extraction vessel where the cobalt salt is converted into oil soluble form and finally after numerous additional steps is used to supply a portion of the catalyst requirements for the oxonation reaction. Such a procedure is complex and inefficient and adds to the operating cost of the process. In addition acetic acid is highly soluble in the organic phase and without additional treatment too much acetic acid passes with the crude aldehyde to the hydrogenation (or hydro) stage. Such “additional treatment”, for instance washing with fresh water, is economically and environmentally unattractive.
Numerous other variations on the Oxo Process are taught, for instance, in U.S. Pat. Nos. 2,638,485; 2,744,936; 2,754,332; 2,757,204; 2,757,206; 2,768,974; 2,812,356; and 3,055,942.
It has been recognized that many forms of cobalt catalyze the oxonation reaction. One preferred method of supplying the catalyst is to employ the oil-soluble Co+2 compounds, such as cobalt naphthenate. After the carbonylation reaction and decobalting of the crude product in the demetalling stage, a catalyst makeup stage is required to convert the Co+2 back to the oil-soluble form. In an attempt to avoid this catalyst makeup stage, U.S. Pat. No. 2,834,815 teaches the use of solid cobalt acetate, preferably added as a slurry with the olefin feed. Water or dilute acetic acid is then used to decobalt the crude aldehyde product and solid cobalt acetate is recovered by evaporation of the aqueous acetate solution. This process, however, provides for low olefin conversion when compared with the use of the oil-soluble cobalt catalysts and is difficult to run such a process, with recycling, in a continuous manner.
In another effort to avoid the catalyst makeup stage but taking what might be considered the opposite approach, U.S. Pat. No. 2,757,205 teaches that under appropriate conditions of temperature and pressure the use of H2 and CO in the demetalling step produces a cobalt carbonyl (Co−1) species that may be recycled to the oxonation reactor. The demetalling step still utilizes the addition of aqueous acetic acid, however the cobalt may be concentrated by distilling off the acid and water. The patent explicitly states that such a concentration step is not possible with cobalt acetate or formate (see col. 4, line 27+, of the patent). U.S. Pat. No. 2,767,217 teaches a variation of this process.
U.S. Pat. No. 4,625,067 describes recovery of cobalt values by contacting the hydroformylation crude product with a stripping gas to entrain more than 60% of the cobalt values as volatile cobalt compounds, in the presence of water or aqueous acid (“Cobalt Flash Process”). After contacting the crude product with the stripping gas (preferably synthesis gas), the cobalt-containing aqueous phase is separated and concentrated in a concentrator, preferably by flashing the aqueous phase in an evaporator. This aqueous phase then only contains a portion of the cobalt values in the hydroformylation product, because more than 60%, typically 70%, and preferentially as much as 80% of the cobalt is entrained with the stripping gas as volatile cobalt. This process preferably uses cobalt formate/formic acid solutions. In practice it has been found that acetic acid is not advantageously used in this process for several reasons, not the least of which being that since acetic acid is so soluble in the organic phase, too much is lost to the system with the extraction of the product in the demetalling stage. U.S. Pat. Nos. 4,410,090, 5,218,134 and 5,237,105 describe further improvements of the Cobalt Flash Process, all of them comprising as an essential step in the cobalt recycle process to hydroformylation a step wherein the cobalt is transferred using a vapor stream carrier like a stripping gas.
Finally, U.S. Pat. No. 6,015,928 teaches a process that combines a precarbonylation stage, where the catalyst is worked up in a form to be supplied to the reactor(s), and the oxonation reaction into a single two-phase reactor, where the Co2+salt is converted into Co−1 compounds and taken up into the olefin phase.
Loss of dissolved cobalt salts and acid from the demetalling and cobalt workup stages continues to be a problem. Lack of efficient recycling techniques require that some portion of cobalt salt and acid is lost to the system and instead adds to the environmental load of the process. In a typical industrial process, cobalt losses via waste water are significant; see, for instance, U.S. Pat. No. 5,130,107.
Thus a more efficient means of recycling cobalt values, acid, and water is clearly desirable.